If you are buying Bitcoin for cash, then you don't want to stand around for 30 minutes or more before you feel confident that it is safe to hand over several hundred pounds.
If you have a trust relationship with the other party, then there is no need to wait for confirmations at all.
If you don't have a trust relationship, then 10 minutes (on average), and less than 10 minutes the majority of the time, is a reasonable amount of time to wait for the ability to engage in a non-reversible trust-free transaction.